A source driver circuit for a Thin Film Transistor-Liquid Crystal Display (TFT-LCD) applies a gradation voltage, e.g., gray scale voltage, corresponding to display data to a display panel through a source line. For example, when a gate driver turns on a switch, the source driver applies the gradation voltage to a liquid crystal capacitor that is connected to the switch. FIG. 1 illustrates a conventional source driver 100, which includes a decoder 110 and an amplifier 120. The decoder receives the gray scale voltages (VGRAY) and outputs a gray scale voltage D_VOL based on the display data D. If the display data D is n bits, then the gray scale voltages VGRAY comprise 2n different voltage levels between a source voltage and a common or ground voltage. The amplifier 120 amplifies the selected gray scale voltage D_VOL and applies an amplifier gray scale voltage VOUT to a display panel.
FIG. 2 is a schematic of an input portion of the amplifier 120 of FIG. 1. The gray scale voltage D_VOL is applied as an input to the gates of transistors NTR1 and PTR1. Based on the level of the gray scale voltage D_VOL, either one of NTR1 and PTR1 is turned on or both NTR1 and PTR1 are turned on. An output driving voltage VOUT is generated at the output node NOUT and is fedback into the gates of NTR2 and PTR2.
FIG. 3 illustrates the regions of operation for transistors NTR1 and PTR1. In operation region C, VSS<D_VOL<Vth of PTR1. In this case, PTR1 is turned on, NTR1 is turned off, IS1 operates, and IS2 does not operate. In operation region B, Vth of PTR1<D_VOL<Vth of NTR1. In this case, PTR1 is turned on, NTR1 is turned on, IS1 operates, and IS2 operates. In operation region A, Vth of NTR1<D_VOL<VDD. In this case, NTR1 is turned on, PTR1 is turned off, IS2 operates, and IS1 does not operate.
FIG. 4 illustrates current consumption based on the particular operation region for transistors NTR1 and PTR1. Region 1 represents the current consumption when the gray scale voltage D_VOL is in region C of FIG. 3. Region 2 represents the current consumption when the gray voltage D_VOL is in region B of FIG. 3. Region 3 represents the current consumption when the gray voltage D_VOL is in region A of FIG. 3. Unfortunately, if the voltage level of the gray scale voltage D_VOL is in region 2 (region B of FIG. 3), the current consumption is about twice that of regions 1 and 3 (regions C and A of FIG. 3).